Solid state imagers such as CCD, CMOS, and others, are in widespread use. CMOS image sensors are increasingly being used as a lower cost alternative to CCDs. A CMOS image sensor circuit includes a focal plane array of pixels, each one of the pixels includes a photogate, photoconductor, or photodiode having an associated charge accumulation region within a substrate for accumulating photo-generated charge. Each pixel may include a transistor for transferring charge from the charge accumulation region to a storage region and another transistor for resetting the storage region to a predetermined charge level prior to charge transfer. The pixel may also include a source follower transistor for receiving and amplifying charge from the storage region and an access transistor for controlling the readout of the pixel contents from the source follower transistor which receives the charge.
FIG. 1 shows one example of a CMOS imaging sensor 1 that includes a CMOS active pixel sensor (“APS”) pixel array 4 and a controller 6 that provides timing and control signals to enable the readout of image signals captured and stored in the pixels in a manner commonly known to those skilled in the art. Example arrays have dimensions of M×N pixels, with the size of the array 4 depending on a particular application. The imager pixel signals, typically in the form of reset signal Vrst and photosensor signal Vsig are read out a row at a time using a column parallel readout architecture. The controller 6 selects a particular row of pixels in the array 4 by controlling the operation of row addressing circuit 2 and row drivers 3. Signals stored in the selected row of pixels are provided on respective column output lines to a readout circuit 7. The signals read from each of the columns are read out sequentially or in parallel using a column addressing circuit 8. The Vrst and Vsig signals are typically subtracted to provide an image signal free from certain types of noise. The readout analog Vrst and Vsig signals for each pixel are subtracted and are converted to digital values by an analog-to-digital (“A/D”) converter 9 and then processed by an image processor 10.
CMOS image sensors of the type discussed above are generally known as discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046 2050 (1996); and Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452 453 (1994). See also U.S. Pat. Nos. 6,177,333 and 6,204,524, which describe operation of conventional CMOS image sensors and U.S. Pat. No. 7,141,841, which describes an imager having extended dynamic range, assigned to Micron Technology, Inc.
FIG. 2 depicts a portion of the FIG. 1 sensor 1 and more details of the image processor 10. Image processor 10 receives digital signals on line 31 from the pixel array 4 and upstream circuits (not shown). The illustrated portion of the image processor 10 is typically constructed as an image processing pipeline. The dotted lines represent omitted circuits performing upstream processing tasks. Typically, the incoming pixel signals, in the form of Bayer pixel data, are received by a digital gain unit 29, which may amplify one or more of the pixel signals, which are then are provided to a PGA (positional gain amplifier) 30 that selectively amplifies the Bayer pixel signals based on their pixel location in the pixel array 4. The digital gain unit 29 amplifies the pixel signals per pre-imposed design or by need. The PGA 30 also applies gains to the image signal per pre-imposed design or by need. The dotted line between the digital gain unit 29 and PGA 30 represents other processing circuits performing other pixel signal processing tasks, which may be provided between the two.
After processing by the PGA 30 and other possible processing circuits, which operate on the Bayer pixel signals, the Bayer pixel signals are demosaiced by a demosaicing unit 31 to provide RGB data for each pixel. The RGB data is subsequently color corrected by a color correction matrix (ccm) 32. Following color correction, other processing may be performed on the RGB pixel data and the color corrected RGB pixel data is then provided to a Gamma correction unit 40. The Gamma correction unit 40 applies linear luminance to the RGB pixel signals. The application of Gamma correction may be one of many different processing steps that are applied to the RGB image pixel signals before being provided downstream for printing, display, and/or storage.
For automatic exposure, statistics of the RGB image can be collected by an exposure statistics processing unit 41 at the output of the demosaic unit 31, processed and provided to an automatic (auto) exposure algorithm 43 for use in determining a proper image exposure setting based on an average image luminance.
Image sensors having a pixel array have a characteristic dynamic range. Dynamic range refers to the range of incident light that can be accommodated by the pixels of the array in a single frame of pixel data. It is desirable to have an image sensor with a high dynamic range to image scenes that generate high dynamic range incident signals, such as indoor rooms with windows to the outside, outdoor scenes with mixed shadows and bright sunshine, night-time scenes combining artificial lighting and shadows, and many others.
The dynamic range for an image sensor is commonly defined as the ratio of its largest non-saturating pixel signal to the standard deviation of noise under dark conditions. The dynamic range is limited on an upper end by the charge saturation level of the photosensors, and on a lower end by noise imposed limitations and/or quantization limits of the analog-to-digital converter used to produce the digital pixel signals. When the dynamic range of an image sensor is too small to accommodate the variations in light intensities of the imaged scene, a captured image may have limited dynamic range.
In conventional imager processing systems, conventional auto exposure techniques attempt to achieve an average image luminance at a certain target level, where the auto exposure target can be established in advance and/or at the time of image capture to best capture the dynamic range of the scene. Additionally, the auto exposure target can be set by the imaging sensor with or without input by the user of the imaging sensor. An advantage of conventional auto exposure techniques based on the average image luminance is that they are easily implemented and can provide fast and smooth behavior.
One of the problems with digital image capture occurs when portions of, or an entirety of, a captured image are not equally lit. Some portions of an image may have lowlights, highlights, or may have backlighting. Another problem is that portions of, or an entirety of, a captured image are not properly lit where the overall image is extremely brightly lit, dimly lit, or has low-contrast content. Conventional techniques relying on average image luminance do not always effectively compensate for an image that lacks uniform luminance or has other problems. As a result, conventional auto exposure techniques relying on average image luminance to determine image exposure may not set the best exposure to adequately capture a full dynamic range of a scene resulting in images that, for example: have a subject that is underexposed due to backlighting; have brightly sun lit outdoor scenes which appear too dark; have night scenes that look too bright; produce dull, low-contrast images; and/or produce images with clipped highlights with reduced dynamic range. Therefore, it is desirable to have an auto exposure technique that at least mitigates some of these problems and better sets exposure to improve the capture of the dynamic range of the scene.